This invention relates to tone decoders in general and particularly to a tone decoder in which multiple correlators are utilized to confirm the presence of a desired tone. A digital tone decoder, such as illustrated in U.S. Pat. No. 4,216,463, is based on a digital quadrature correlation technique. The frequency response can approximately be described by the equation ##EQU1## where; T is the observation time in seconds
f.sub.c is the center frequency in Hertz PA1 f.sub.i is the incoming frequency in Hertz. PA1 A=.pi. T (f.sub.c -f.sub.i).
The bandwidth of the correlator is defined as the range of frequencies that will cause the correlation value to be above a detect threshold. It will readily be appreciated that the bandwidth is inversely related to the observation time. Consequently, with such an approach if a relatively narrow bandwidth is required, the observation time must be relatively large, and any decrease in the observation time will result in a wider bandwidth.
It is often desirable to provide some detector that include both relatively narrow bandwidth and a relatively short observation time in order to provide for fast detecting of a tone which may be close in frequency to other possible tones.